Field of Invention
Various embodiments of the present disclosure are directed to an optical phase diversity receiver, and more particularly, to an optical phase diversity receiver for coherent optical communication which separates, depending on polarization characteristics and a phase difference, light combined for optical communication.
Description of Related Art
With regard to the development of the optical communication technology, a high-speed information and communication device using coherent technology is developing from an OOK (on-off keying) direct amplitude modulation method of modulating the intensity of light to a phase shift keying (PSK)-phase modulation method of modulating the phase of an optical signal or a method of simultaneously modulating the phase and the light intensity.
The coherent optical communication uses a phase component of an optical signal in data processing and increases the capacity of data processing through phase modulation, polarization modulation, intensity modulation and wavelength division methods. With regard to the coherent optical communication technology, a receiver which receives polarization-combined/phase-modulated signals and separates polarized light and divides the phases into I and Q signals is called a phase diversity receiver.
The phase diversity receiver combines, to amplify a received optical signal, it with an internal optical signal, which is called a local oscillator signal, so as to increase the intensity of light, and thereafter separates only phase components and restores the signal. To separate the phase components, if from respective I and Q components, signals having 180° phase differences (0° and 180°, 90° and 270°) are converted into electrical signals by an optical detector, and the two signals are removed by a differential amplifier or the like, only phase component signals can be obtained.
That is, the phase diversity receiver performs 1) combination of a received optical signal and a local oscillator optical signal, 2) TE (transverse electric)/TM (transverse magnetic) polarization separation of the optical signal, 3) I and Q component separation of the optical signal, 4) I (0°), I-bar (180°), Q (90°) and Q-bar (270°) component separation of the optical signal, 5) conversion of the optical signal into an electric signal, 6) detecting phase components from the electric signal, and 7) data processing of the detected phase components. Of them, 1) to 4) are operations of restoring the signal using light and are called “optical hybrid”, and 5) to 7) are operations of restoring the signal using an electric signal.
FIG. 1 is a block diagram showing the concept of an optical hybrid of a conventional optical phase diversity receiver. An optical signal S and a local oscillator optical signal LO are received to the optical phase diversity receiver. The optical signal S and the local oscillator optical signal LO both have TE and TM components.
The received optical signal S is separated into a TE component and a TM transverse magnetic component by a first polarization separator 11. The TE component of the optical signal S is distributed in power by a first optical distributor 21, and the distributed components are respectively transmitted to first and second couplers 41 and 42. The TM component of the optical signal S is distributed in power by a third optical distributor 23, and the distributed components are respectively transmitted to third and fourth couplers 43 and 44.
The local oscillator optical signal LO for amplifying the optical signal S is separated into a TE component and a TM component by a second polarization separator 12. The TE component of the local oscillator optical signal LO is distributed in power by a second optical distributor 22, and the distributed components are respectively transmitted to the first and second couplers 41 and 42. The TM component of the local oscillator optical signal LO is distributed in power by a fourth optical distributor 24, and the distributed components are respectively transmitted to the third and fourth couplers 43 and 44. The local oscillator optical signal LO having the TE component is shifted by 90° by a first phase shifter 31 while it is transmitted to the second coupler 42. The local oscillator optical signal LO having the TM component is shifted by 90° by a second phase shifter 31 while it is transmitted to the fourth coupler 44.
Transmitted to the first coupler 41, the optical signal S having the TE component and the local oscillator optical signal LO having the TE component are combined with each other by an interference phenomenon, and the combined optical signal is separated into I and I-bar components with a phase difference of 180° and outputted from first and second output terminals 51 and 52 of the first coupler. Transmitted to the second coupler 42, the optical signal S having the TE component and the local oscillator optical signal LO having the TE component are combined with each other by an interference phenomenon, and the combined optical signal is separated into Q and Q-bar components with a phase difference of 180° and outputted from first and second output terminals 53 and 54 of the second coupler.
Transmitted to the third coupler 43, the optical signal S having the TM component and the local oscillator optical signal LO having the TM component are combined with each other by an interference phenomenon, and the combined optical signal is separated into I and I-bar components with a phase difference of 180° and outputted from first and second output terminals 55 and 56 of the third coupler. Transmitted to the fourth coupler 44, the optical signal S having the TM component and the local oscillator optical signal LO having the TM component are combined with each other by an interference phenomenon, and the combined optical signal is separated into Q and Q-bar components with a phase difference of 180° and outputted from first and second output terminals 57 and 58 of the fourth coupler.
The conventional optical phase diversity receiver having the above-mentioned configuration is problematic in that because it includes, to embody the optical hybrid, the polarization separator, the plurality of optical distributors, the couplers, the phase shifter and the optical waveguides connecting these components, the size thereof is comparatively large, and the production cost is increased.